The invention relates to a crucible having multi-layered walls. More particularly, the instant invention relates to a crucible especially adapted for receiving a melt for drawing crystals, in particular of single crystals according to the Czochralski process.
The Czochralski process is well known and has been described in many texts dealing with the subject of crystal growth, for example "The Growth of Crystals" Prentice Hall, Inc. 1970, page 174 ff. Library of Congress Catalog Card No. 77-104173.
A requirement for such a crucible is that it be resistant to attack by the high melting point melts normally used in this process as well as that it be resistant to oxygen or oxidizing substances at high temperatures.
Crucibles normally used for this purpose are composed of a platinum metal; for example, iridium, with a melting point of 2,454.degree. C., has been found particularly useful. In general, a melt is prepared in an iridium crucible at about 1800.degree. C. and thereafter a seed crystal is introduced. The seed crystal is slowly withdrawn from the melt, under controlled conditions of temperature and withdrawal velocity, and a cylindrical crystal is thereby formed on the seed crystal.
The disadvantage of conventional iridium crucibles lies in their relatively short useful life and the high cost of replacing or remaking the iridium crucible. Iridium is expensive and must be processed at temperatures in the order of 1200.degree. C.
The short usable life of the iridium crucible is a result of corrosion by the melt, which becomes progressively more rapid as the surface area of the inside of the crucible increases due to the roughening effect of corrosion.
The rate of corrosion is highly dependent on the temperature of the crucible. Corrosion rates increase with increasing temperature. As it is common to heat such crucibles by means of high frequency induction coils, the problem of corrosion is increased due to hot spots or uneven temperature gradients which occur across the normally non-homogenous crucible as a result of heating by induction.
When the corrosion progresses to the point that the crucible can no longer function properly, the crucible is subjected to processing: the iridium content of the crucible is reclaimed and remolded into a new crucible. As noted above, the iridium can be processed only at a temperature of about 1200.degree. C. and the cost of replacing the iridium lost by corrosion is high.
Desirable features for a crucible include a minimum use of expensive iridium and a reduction of the losses of the iridium. Additionally, it is desirable to protect the melt from contamination from crucible materials.